1. Field
This invention pertains to the storage of hydrogen in the form of solid metal hydrides contained in appropriate containers, e.g., pressure vessels. In particular, the invention relates to methods and apparatus for transferring heat to and from the metal hydrides contained in the storage containers.
2. State of the Art
One factor that has limited the use of hydrogen, especially as a fuel, is the difficulty of efficiently and safely storing it. Storage as a liquid is costly due to the energy expended in liquifying the hydrogen, and the extremely low temperature of the liquid hydrogen presents numerous safety problems. Storing hydrogen as a gas requires extremely heavy and bulky containers and is impractical for many contemplated uses of hydrogen.
An attractive alternative to the conventional storage methods has been recently proposed in which hydrogen is stored in the form of a metallic hydride. Many metals and alloys will reversibly react with hydrogen to form metallic hydrides which contain more hydrogen per unit volume than liquid hydrogen. Heat is liberated when the hydrogen and metallic material reacts to form the hydrides and must be removed to allow the hydriding reactions to proceed to completion. Conversely, heat is absorbed during the decomposition of the hydride to release hydrogen, and the hydrides are preferably heated during their decomposition to provide an adequate rate of liberation of hydrogen therefrom.
Heating and cooling of the metallic hydride material has been accomplished by conventional techniques including heating or cooling the container in which the material is held, or spacing tubes throughout the bed of hydride material and circulating a heat exchange medium in the tubes. In such techniques, the amount of heat transferred to the metallic hydride depends on the area of the container or the surface area of the tubes extending through the bed, as well as on the conductive heat transfer characteristics of the metallic hydride. It has also been suggested to use hydrogen gas itself as a convective energy carrier, and, thus, overcome the limitations of the above-mentioned techniques. In addition, the direct cooling and heating of the hydrides permits rapid cycling between charge and discharge operations, and, thus, increase the efficiency of a given system. As proposed in paper number 760569 presented at the SAE Fuels and Lubricants Meeting in St. Louis, Mo., June 7-10, 1976, by Hoffman et al. of Brookhaven National Laboratory, hydrogen would be circulated through the metallic hydride in the containers to carry heat directly to where it is needed. Heat exchange would take place with the hydrogen in an external heat exchanger to supply the heat to the hydrogen. Large compressors would be required to circulate the hydrogen. Such compressors are high capital cost items and are expensive to operate due to high maintenance and energy requirements.